EQUIVARIANT STABLE HOMOTOPY THEORY J.P.C. GREENLEES AND J.P. MAY Contents Introduction 1 1. Equivariant homotopy 2 2. The equivariant stable homotopy category 10 3. Homologyandcohomologytheoriesandfixedpointspectra 15 4. Change of groups and duality theory 20 5. Mackey functors, K(M,n)’s, and RO(G)-gradedcohomology 25 6. Philosophy of localization and completion theorems 30 7. How to prove localization and completion theorems 34 8. Examples of localization and completion theorems 38 8.1. K-theory 38 8.2. Bordism 40 8.3. Cohomotopy 42 8.4. The cohomology of groups 45 References 46 Introduction The study of symmetries on spaces has always been a major part of algebraic and geometric topology, but the systematic homotopical study of group actions is relatively recent. The last decade has seen a great deal of activity in this area. After giving a brief sketch of the basic concepts of space level equivariant homo- topy theory, we shall give an introduction to the basic ideas and constructions of spectrum level equivariant homotopy theory. We then illustrate ideas by explain- ing the fundamental localization and completion theorems that relate equivariant to nonequivariant homology and cohomology. The first such result was the Atiyah-Segal completion theorem which, in its simplest terms, states that the completion of the complex representation ring R(G) at its augmentation ideal I is isomorphic to the K-theory of the classifying space ∧ ∼ BG: R(G)I = K(BG). A more recent homological analogue of this result describes 1 2 J.P.C. GREENLEES AND J.P. MAY the K-homology of BG. As we shall see, this can best be viewed as a localization theorem. These are both consequences of equivariant Bott periodicity, although full understanding depends on the localization away from I and the completion at I of the spectrum KG that represents equivariant K-theory. We shall explain a still more recent result which states that a similar analysis works to give the same kind of localization and completion theorems for the spectrum MUG that represents a stabilized version of equivariant complex cobordism and for all module spectra over MUG. We shall also say a little about equivariant cohomotopy, a theory for which the cohomological completion theorem is true, by Carlsson’s proof of the Segal conjecture, but the homological localization theorem is false. 1. Equivariant homotopy We shall not give a systematic exposition of equivariant homotopy theory. There are several good books on the subject, such as [12] and [17], and a much more thor- ough expository account will be given in [53]. Some other expository articles are [49, 1]. We aim merely to introduce ideas, fix notations, and establish enough back- ground in space level equivariant homotopy theory to make sense of the spectrum level counterpart that we will focus on later. The group. We shall restrict our attention to compact Lie groups G, although the basic unstable homotopy theory works equally well for general topological groups. To retain the homeomorphism between orbits and homogenous spaces we shall always restrict attention to closed subgroups. The class of compact Lie groups has two big advantages: the subgroup struc- ture is reasonably simple (‘nearby subgroups are conjugate’), and there are enough representations (any sufficiently nice G-space embeds in one). We shall sometimes restrict to finite groups to avoid technicalities, but most of what we say applies in technically modified form to general compact Lie groups. The reader unused to equivariant topology may find it helpful to concentrate on the case when G is a group of order 2. Even this simple case well illustrates most of the basic ideas. G-spaces and G-maps All of our spaces are to be compactly generated and weak Hausdorff. A G-space is a topological space X with a continuous left action by G; a based G-space is a G-space together with a basepoint fixed by G. These will be our basic objects. We frequently want to convert unbased G-spaces Y into based ones, and we do so by taking the topological sum of Y and a G-fixed basepoint; we denote the result by Y+. EQUIVARIANT STABLE HOMOTOPY THEORY 3 We give the product X × Y of G-spaces the diagonal action, and similarly for the smash product X ∧ Y of based G-spaces. We use the notation map(X, Y ) for the G-space of continuous maps from X to Y ; G acts via (γf)(x)= γf(γ−1x); we let F (X, Y ) denote the subspace of based maps. The usual adjunctions apply. A map of based G-spaces is a continuous basepoint preserving function which commutes with the action of G. A homotopy of based G-maps f0 ≃ f1 is a G-map X ∧ I+ −→ Y whose composites with the inclusions of X ∧{0}+ and X ∧{1}+ are f0 and f1. We use the notation [X, Y ]G to denote the set of homotopy classes of based G-maps X −→ Y . Cells, spheres, and G-CW complexes We shall be much concerned with cells and spheres. There are two important sorts of these, arising from homogeneous spaces and from representations, and the interplay between the two is fundamental to the subject. Given any closed subgroup H of G we may form the homogeneous space G/H and its based counterpart, G/H+. These are treated as 0-dimensional cells, and they play a role in equivariant theory analogous to the role of a point in nonequivariant theory. We form the n-dimensional cells from these homogeneous spaces. In the unbased context, the cell-sphere pair is (G/H × Dn, G/H × Sn−1), and in the based context n n−1 (G/H+ ∧ D , G/H+ ∧ S ). We shall always use different notation for different actions, so that when we write Dn and Sn we understand that G acts trivially. Starting from these cell-sphere pairs, we form G-CW complexes exactly as non- equivariant CW-complexes are formed from the cell-sphere pairs (Dn,Sn−1). The usual theorems transcribe directly to the equivariant setting, and we shall say more about them below. Smooth compact G-manifolds are triangulable as finite G-CW complexes, but topological G-manifolds need not be. We also have balls and spheres formed from orthogonal representations V of G. We shall be concerned especially with the one-point compactification SV of V , with ∞ as the basepoint; note in particular that the usual convention that n denotes the trivial n-dimensional real representation gives Sn the usual meaning. We may also form the unit disc D(V )= {v ∈ V |kvk≤ 1}, and the unit sphere S(V )= {v ∈ V |kvk =1}; 4 J.P.C. GREENLEES AND J.P. MAY we think of them as unbased G-spaces. There is a homeomorphism SV =∼ D(V )/S(V ). The resulting cofibre sequence V S(V )+ −→ D(V )+ −→ S can be very useful in inductive arguments since there is an equivariant homotopy 0 equivalence D(V )+ ≃ S . Fixed points and quotients. There are a number of ways to increase or decrease the size of the ambient group. If f : G1 −→ G2 is a group homomorphism we may regard a G2-space Y as a G1- space f ∗Y by pullback along f, and we usually omit f ∗ when the context makes it clear. The most common cases of this are when G1 is a subgroup of G2 and when G2 is a quotient of G1; in particular every space may be regarded as a G-fixed G-space. The most important construction on G-spaces is passage to fixed points: XH = {x ∈ X | hx = x for all h ∈ H}. For example, F (X, Y )G is the space of based G-maps X −→ Y . It is easy to check that the fixed point spaces for the conjugates of H are all homeomorphic; indeed, −1 multiplication by g induces a homeomorphism g : Xg Hg −→ XH . In particular H X is invariant under the action of the normalizer NG(H), and hence it has a natural action of the Weyl group WG(H) = NG(H)/H. Passage to H-fixed point spaces is a functor from G-spaces to WG(H)-spaces. Dually, we have the quotient space X/H of X by H. This is actually a standard abuse of notation, since H\X would be more consistent logically; for example, we are using G/H to denote the quotient of G by its right action by H. Again, multiplication by g gives a homeomorphism X/g−1Hg −→ X/H. Thus X/H also has a natural action of the Weyl group, and passage to the quotient by H gives a functor from G-spaces to WG(H)-spaces. If N is a normal subgroup of G, then it is easy to verify that passage to N- fixed points is right adjoint to pullback along G −→ G/N and that passage to the quotient by N is left adjoint to this pullback. Lemma 1.1. For G-spaces X and G/N-spaces Y , there are natural homeomor- phisms G-map(Y,X) =∼ G/N-map(Y,XN ) and G/N-map(X/N, Y ) =∼ G-map(X, Y ), and similarly in the based context. The particular case G-map(G/H,X) =∼ XH helps explains the importance of the fixed point functor. EQUIVARIANT STABLE HOMOTOPY THEORY 5 Isotropy groups and universal spaces. An unbased G-space is said to be G-free if XH = ∅ whenever H 6= 1. A based G-space is G-free if XH = ∗ whenever H 6= 1.